US State Department officials offered their most detailed description yet of the dramatic events in Benghazi that led to the death of a US ambassador, but they backed away from earlier assertions that the events were triggered by protests against an anti-Islam video.

The officials were briefing reporters on the eve of a congressional hearing into on the attack last month, which is expected to focus on security missteps by the department.

They described frantic and prolonged efforts to rescue Ambassador Christopher Stevens from a smoke-filled "safe haven" inside the US diplomatic compound in Benghazi where he apparently died of asphyxiation.

Stevens' death and confusion over the attack has become the subject of fierce partisan debate in Washington in the final weeks before the US presidential election on November 6.

The State Department officials said agents crawled on their hands and knees through thick diesel smoke to try to find the missing envoy, who somehow was transported out of the compound to a local hospital.

The US government learned where he was after someone called numbers in his cell phone, the officials said.

"We do not know exactly how the ambassador got to the hospital. That is one of the issues that we hope to resolve in the ongoing reviews, and the information we are still seeking," one official said.

The officials also said there was "nothing unusual" around the Benghazi mission before the assault. Earlier accounts by White House and State Department officials, including US Ambassador to the United Nations Susan Rice, suggested that the attacks were triggered by protests over an anti-Muslim video made in California that insulted the Prophet Mohammad.

FOCUS ON DIPLOMATIC SECURITY

Officials of the State Department's Bureau of Diplomatic Security will testify at a House of Representatives hearing on Wednesday (local time) and one key subject of the inquiry will be whether the State Department rejected requests from diplomats to increase security at the Libya mission after months of violent incidents.

One senior official described the Benghazi attack as unprecedented and said security measures were always being adjusted. "We attempt to mitigate our risks. We cannot eliminate them," the official said.

The officials described the rented villa in which Stevens was hiding as a large residence with numerous bedrooms. Half of one floor was a "safe haven" barricaded with a gate and locks.

Stevens, Sean Smith, an information management officer, and five armed American security agents were in the compound the night of attack on September 11. There were also four members of a Libyan militia, assigned as the local government's protection force.

Stevens arrived in Benghazi on September 10 and the next day held a series of meetings at the compound. His last visitor was a Turkish diplomat, whom he escorted to the main gate at 8:30pm local time, a State Department official said.

"There had been nothing unusual during the day at all outside," the official said.

The State Department officials downplayed earlier assertions that the anti-Muslim film was a trigger for the violence. "That is the question that you would have to ask others. That was not our conclusion, that's not saying we had a conclusion, but we outlined what happened," one official said.

GUNFIRE AND AN EXPLOSION

At 9:40pm, security agents in Benghazi heard loud noises at the gate, gunfire and an explosion. A large number of armed men entered the compound.

One agent went to fetch the ambassador from his bedroom as well as Smith. The three entered the so-called safe haven, which had window grills and a central windowless closet area where people could take refuge.

The security agent was armed with a submachine gun and a sidearm. He radioed to other agents that he was with Stevens in the safe haven.

Other agents tried to enter the villa, but they encountered a large group of armed men and retreated to another building in the compound where they barricaded themselves in.

The attackers swarmed into the darkened villa and walked around in the living area. They looked through the grill into the safe area and tried to enter it but could not.

The agent protecting Stevens watched their movements with a gun trained on them, ready to shoot.

The attackers carried cans of diesel fuel that they sprinkled on furniture and set on fire.

The building filled with smoke and fumes, and the air inside grew black. Stevens, Smith and the security agent moved to a bathroom in the safe area where they opened a window but still could not get enough air.

They decided to leave through an adjacent bedroom. Outside, there were shots, tracer bullets, smoke and explosions.

The officials said the security agent, whom they did not identify, was suffering "severely" from smoke inhalation and could barely breathe. He left the villa first, following protocol, but when he turned back he did not see the other two.

OVERCOME BY SMOKE

He returned to try to rescue Stevens but he could not find him. He went in and out of the building several times before he was overcome by smoke.

The agent went up a ladder to the roof, collapsed and radioed other agents who arrived to continue the hunt for Stevens and Smith.

"They take turns going into the building on their hands and knees, feeling their way through the building to try to find their two colleagues. They find Sean. They pull him out of the building. He is deceased. They are unable to find the ambassador," one official said.

Six security personnel from a US annex nearby arrived with members of the Libyan militia, known as the February 17 Brigade.

They took people from the compound and transported Smith's body to a secure annex, running into traffic, hand grenades, and two flat tires.

The annex came under fire, killing two security personnel and wounding another. They spent hours securing the annex, then evacuated everyone on two flights.

There were no classified materials that had to be secured at the mission site where Stevens had been, the official said.

Asked whether anyone had counselled Stevens against going to Benghazi on September 11 - the 11th anniversary of the 2001 attacks on the United States - the official said: "Ambassadors must travel; ambassadors must get out and meet with a variety of individuals especially in countries that have multiple centres of energy or power. This just must happen."